Load indicating fasteners

ABSTRACT

A load indicating fastener composed of a body (2) having an abutment (9) which is moved by the changing length of the body (2) under an applied load, and a load sensing member (10) composed of an element (11) disposed in the direction in which a load is applied to the body (2) when the fastener is in the user characterized in that the element (11) is of elongated U-shape, a first limb (13) of the U providing an anchorage part (14) which is rigidly anchored to the body (2), the closed end of the U providing a bearing part (15) which bears on the abutment (9) so as to move with the abutment relative to the anchorage part (14) in response to a load applied to the body, and a second limb (16) of the U having an indicator (17) at its outer end which responds movement of the bearing part (15) and registers the applied load related to that movement on a scale (19).

BACKGROUND OF THE INVENTION

This invention relates to a load indicating fastener which is adapted toindicate loads to which it is subjected in use.

It is desirable that applied fasteners should be properly tightened todesigned load levels in order to ensure that secure joints are achievedby the use of the fasteners. Torque wrenches are commonly used fortightening fasteners such as bolts to pre-determined loads. They measureapplied effort and are subject to friction so that the fastener loadsthey produce may not be as accurate as they might be. Fasteners havebeen proposed which indicate when desired loads have been applied toitems but such fasteners have tended to be expensive and/or complicatedto install.

One form of load indicating fastener is known from French Patent No.1,477,006 in which a graduated wire or strip is fixed at one end in aninternal bore of a stud. Elongation of the stud under tensile stress ismeasurable against the fixed scale of the strip to provide an indicationof applied load. In this arrangement it is difficult to accurately readthe small elongation against the scale.

Another form of fastener is disclosed in UK Patent Application GB2,212,284 in which a stem fixed at one end to the shank of a boltcarries an electrical contact at its other end. Extension of the shankunder an applied load draws the electrical contact towards an axiallyfacing surface of the bolt. The variation in capacitance between theelectrical contact and the surface may be used to provide an indicationof the bolts extension and, hence, the applied load. Alternatively,completion of an electrical circuit between the electrical contact andthe surface may indicate that a predetermined load has been applied.This system has been found to be complicated and expensive to implementwith the required degree of accuracy. In addition, it does not provide avisual indication of applied load.

SUMMARY OF THE INVENTION

The present invention aims to provide a load indicating fastener whichindicates applied loads effectively and may be manufactured atreasonable cost.

According to the present invention a load indicating fastener isprovided which comprises a body having an abutment which is moved by thechanging length of the body under an applied load, and load sensingmeans comprising an element disposed in the direction in which a load isapplied to the body when the fastener is in use characterised in thatthe element is of elongated U-shape, a first limb of the U providing ananchorage part which is rigidly anchored to the body, the closed end ofthe U providing a bearing part which bears on the abutment so as to movewith the abutment relative to the anchorage part in response to a loadapplied to the body, and a second limb of the U having an indicator atits outer end which responds to movement of the bearing part andregisters the applied load related to that movement on a scale.

The fastener may be in the form of a bolt or a stud, but it may possiblytake other forms. In the form of a bolt the body of the fastenercomprises a head and shank which may be of substantially standard form.Similarly in the form of a stud the body comprises a shank which may beof substantially standard form. In either case the load sensing meansresponds to change in the length of the shank under applied loads.Conveniently the scale is at the bolt head or end of the stud shankwhere it can be seen readily when the fastener is in use.

The load sensing means responds to changes in the length of the body asthe fastener is being secured and when the fastener has been secured foruse. Thus as the fastener is being secured the applied loads will beregistered on the scale which will indicate when a required working loadhas been applied. When the fastener has been secured any change in theload at which it was set for use will be indicated at the scale.

Actual load figures may be indicated on the scale; loads in terms offractions or percentages of a maximum load that should be applied may berepresented, or just one required applied load, or the scale mayindicate loading in some other desired manner. What the scale indicatesmay be determined by the intended use of the fastener.

The load sensing means may be arranged to register applied loads up tothe proof load for the material of the fastener, that is the maximumload that can be sustained before there is a permanent change from theoriginal length of the body.

Preferably the element is made of the same material, or at least has thesame coefficient of thermal expansion, as the material of the body sothat the functioning of the load sensing means will not be affected bychanging temperatures at which the fastener may be used. In a preferredembodiment, the element may be made of wire and the anchorage part is atthe end of one limb remote from the bearing part of the element and thelimb allows the bearing part to move relative to the anchorage part.Preferably the limbs can move resiliently relative to one another aboutthe bearing part in response to movement of the bearing part with theabutment.

At least in part the load sensing means may be located in a blind boreor passage in the body. When the load sensing means comprises anelement, as described above, the abutment on which the bearing partbears may be the blind end of the bore or passage, or it may be providedby an insert fitted into the blind end portion of the bore or passage.The anchorage part may be anchored in or adjacent a mouth of the bore orpassage. The bore or passage may have an enlarged mouth portion in whichthe anchorage part is anchored. Conveniently the scale is located in oradjacent the mouth of such a bore or passage.

There may be direct anchorage of the anchorage part to the body, forexample by welding or adhesive bonding, or the part may be anchored to asupport which is suitably fixed to the body. The support may be in theform of a plug or other closure which closes the bore or passage inwhich the element extends. The scale may be on the support. In thepreferred form of the element the limb having the indicator may passthrough an opening in the support to the scale and be guided by theopening for movement of the indicator relative to the scale.

If desired the load sensing means may be adapted to give a signal, inaddition to registering with the scale, that a given load or loads hasor have been applied to the shank. For example, the load sensing meansmay be contained in an electrical circuit which is designed to producean audible and/or visual signal when a given load is registered on thescale. The electrical circuit could be arranged to operate suitablemeans for setting or adjusting the load on the body of the fastener, ifappropriate.

The load sensing means may be arranged to operate over a given range ofchanges of length of the body under applied loads. This will usually bethe elongation of the body from the unloaded condition up to applicationof the relevant proof load. In the form in which the load sensing meanscomprises an element with a bearing part which bears on an abutment atthe body, the element may be set to bear on the abutment when the bodyis unloaded with a pre-determined bearing force which reduces to zerowhen the relevant proof load is applied. For a given elongation under aproof load a required original, length of the element can be calculatedfrom: ##EQU1## where E=Youngs Modulus for the material of the body.

A=cross sectional area of the body.

For different cross sectional areas and/or proof loads the requiredoriginal length will vary. For a range of fastener sizes and proof loadsload sensing means with elements of different lengths may be provided,as required. However, it is generally easier and more economical forproduction to provide load sensing means for fasteners in accordancewith the invention with elements of a standard length and to compensatefor the difference between that length and the required length for anyparticular fastener by adjusting the position of the abutments at thebodies of the fasteners. Thus where the element extends in a blind boreof passage, as described, the abutment adjustments may be readilyachieved by fitting an insert of the required length difference into thebore or passage for the bearing part of the element to bear upon.

It is possible for the load indicating fastener now provided to give ahighly accurate indication of applied loads. The indication on the scaleof an applied load enables a person to see readily what the appliedloading is on the fastener when it is in use.

The fastener may be manufactured without being substantially moreexpensive than a conventional fastener of similar type. Furthermore, aconventional fastener may be readily adapted to include the load sensingmeans to be in accordance with the present invention.

BRIEF DESCRIPTION OF THE DRAWING

An embodiment of the present invention will now be described by way ofexample with reference to the accompanying drawings in which:

FIG. 1 is a partly sectioned side view of a load indicating fastener inaccordance with the invention shown fitted for use;

FIG. 2 is a plan view of the fastener;

FIG. 3 is an elevational view of a modified element which may beincluded in the fastener, and

FIG. 4 is a plan view of the modified element.

FIG. 5 is a partly sectioned side view, similar to FIG. 1, showing aload indicating fastener in accordance with the invention equipped withan electric circuit to provide a loading warning.

In this embodiment a load indicating fastener is provided in the form ofa bolt 1 have a metal, for example steel, body 2 comprising a hexagonalhead 3 and a shank 4 which has an externally screw-threaded portion 5spaced from the head 3 by a plain cylindrical portion 6.

Drilled into the body 2, co-axially with its rotational axis, is a blindbore 7 which extends from the top of the head 3, through the head andinto the shank 4 for approximately half the length of the plaincylindrical portion 6. The outer end of the bore 7 is counterbored toform an enlarged mouth 8. The closed inner end of the bore 7 serves asan abutment 9.

Fixed in the bore 7 is a load sensing means 10 which comprises anelement 11 made from wire of the same material as the body 2 bent to anarrow elongated U-shape and supported by a metal plug 12. The plug 12is rigidly secured in the mouth 8 of the bore 7, for example by asuitable adhesive. A first limb 13 of the element 11, which in the freestate is straight, has an anchorage part 14 at its outer end; the closedend of the U-shape of the element forms a bearing part 15 of theelement, and the other, second, limb 16 of the element has its outer endbent over at right angles to the limb, in a direction at right anglesaway from the first limb 13 to form an indicator 17. The anchorage part14 of the first limb 13 is anchored in the plug 12, being received intoand bonded in a drilling of complementary diameter in the plug. Thesecond limb 16 passes freely through a slot 18 in the plug extendingtowards the anchorage part 14 and the indicator 17 is exposed outsidethe plug, at the top of the head 3 of the body 3.

Fixed, or marked, on the plug 12 alongside the slot 18 is a scale 19which the indicator overlies. The scale 19 is graduated to show loadsapplied to the shank 4 when the fastener is in use. A protectivetransparent cover 20, of a suitable plastics material or glass, issecured to the head 3, as by adhesive, over the scale 19 and indicator17. If the fastener is to be used at high temperatures the cover 20 maybe made of a temperature resistant glass.

As a modification the head 3 may have a second counterbore to receivethe cover 20 so that the cover is contained within, or substantiallywithin, the head.

The diameter of the bore 7 and the width of the element 11 across itslimbs 13,15 are so related that the element extends with clearance alongthe bore. In the free, unloaded, condition of the element the indicator16 is at the end of the slot 18 remote from the anchorage part 14 of theelement.

The element responds to loads applied to the shank 4 of the body 2 whenthe fastener is in use. The response is related to the extension of theshank under applied loads. It is appropriate for the element to senseand indicate loads applied up to the proof load for the material of thebody. Therefore the element is arranged to respond to extension of theshank up to the extension caused by the proof load. For this to beachieved the element is set in the bore 7 of the body such that thebearing part 15 bears on the abutment and the first limb 13 is under acompressive load which, in the unloaded condition of the fastener,causes that limb to bow between the plug and the abutment to an extentwhich deflects the bearing part towards the plug by a distancecorresponding to the extension of the shank under the proof load.Typically for steel the extension is of the order of 0.2 mm. Thenecessary free length of the first limb 13 from the plug to the bearingpart to provide that deflection is: ##EQU2## E=Youngs Modulus for thebody material A=cross sectional area of the shank

elongation=elongation under proof load,

Load=proof load for the body material

Typically the free length of the first limb 13 is 30-50 mm.

Under the deflection of the bearing part 15 and the bowing of the firstlimb 13 the second limb 16 of the element is resiliently deflectedtowards the first limb. As initially set in the unloaded fastener,therefore, the indicator 17 of the element 11 is at the end of the slot18 nearest to the anchorage part 14 of the element.

As load is applied to the fastener in use and the shank extends underthe load, the bearing force of the bearing part 15 on the abutment 9gradually reduces so that the first limb 13 progressively returnstowards its normal straight form and the second limb 16 moves away fromthe first limb, the indicate 17 moving along the slot 18 away from theanchorage part 14. When the proof load is reached and the bearing forceof the bearing part in the abutment is zero the first limb has fullystraightened, the second limb has moved its full extent away from thefirst limb and the indicator is at or near the opposite end of the slot18 from the anchorage part.

The scale is graduated to show applied loading up to the proof load sothat the indicator registers on the scale loads as they are applied tothe fastener in use. Because of the linear relationship betweenextension and applied load up to the proof load graduating the scalebetween the extreme positions of the indicator is straightforward. Itcan readily be seen, therefore, when a required load has been applied asthe fastener is being secured. The load can be subsequently checked bythe indicator reading on the scale and any deviation from a required setload can be corrected.

In FIG. 1 of the accompanying drawings the fastener is shown in usesecuring two flat members 21,22 together face to face. The shank 4passes through plain holes 23 in the flat members and is fitted withwashers 24,25 respectively between its head 3 and the adjacent member 21and between a nut 26 fitted to the screw-threaded portion 5 of the shankand the other member 22. As the nut 26 is tightened on the shank 4 theapplied tensile load on the shank is registered by the indicator on thescale 19 at the head 3 of the fastener.

Instead of the abutment 9 formed by the closed end of the bore 7, anabutment for the bearing part 15 of the element may be provided by anouter end of an insert 27 located in the bore at its closed end, asindicated in FIG. 1. The insert 27 may conveniently be a metal slug ofthe same material as the body of the fastener. The slug may be cut froma bar or rod of complementary diameter to the bore 7. By the use ofinserts the element may be made of a standard length which may be usedfor the fastener in a range of sizes and the difference in gauge lengthsrequired for the different fastener sizes can be made up by inserts ofthe appropriate lengths.

A modified form of the element 11 is shown in FIGS. 3 and 4 of thedrawings. In this case an outer end portion of the first limb 13 ishelically wound into a closed coil anchorage part 28 through which thesecond limb 16 freely extends. The coiled anchorage part 28 avoids theneed for the plug 12 of the embodiment described above and is fixeddirectly in the mouth 8 of the bore 7. For this element the scale 19 isprovided on a disc, not shown, which is secured on the head of thefastener body or in the, or a further, counterbore of the bore at themouth 8, the disc, like the plug, having a slot for the second limb 16to pass through to the indicator 17.

If desired the element 11, in either of the two forms described, may becoated with a suitable electrical insulating material except at itsanchorage part and indicator. One or more electrical contact points maybe provided on the scale 19. The element and scale may then beincorporated into an electrical circuit to a visual or audible alarmdevice, the circuit, when energised, being arranged to be completed tooperate the device when the indicator makes contact with the, or any,contact point on the scale so as to give a warning that a required load,or certain loads, has or have been applied to the fastener. Thiselectrical circuit could be used to control a tool for tightening thefastener or to give a warning that loading on the secured fastener hasvaried from a required setting and needs to be corrected.

The provision of the load sensing means in the fastener described,including the forming of the bore 7, does not reduce the strength of thefastener because all that is required is contained within the uniformstress portion of the body--the head and unthreaded part of the shank,away from the screw-threaded portion.

An example is shown in FIG. 5 of a modification of the bolt 1 to includean electric circuit which provides a loading warning. The bolt ismodified by replacement of the plug 12 by an electrical connector 29 andwasher 30, both of electrically insulating material, fixed in theenlarged mouth 8 of the bore 7 in the body 2 of the bolt. The first limb13 of the element 11 is anchored at its outer end in the connector 29,and the outer end of the second limb 16 extends through a centralopening 31 in the washer 30 into a slot 32 in the connector 29 alongsidewhich is the scale 19. Located in one end of the slot 32, at the proofload end of the scale 19, is an electrical contact 33. A furtherelectrical contact 34 carried by the connector 29, outside the slot,engages with the head 3 of the bolt. The two contacts 33, 34 areconnected into an electric circuit 35 containing an indicator bulb 36and powered by a battery 37, or possibly by mains supply. The bulb 36may be replaced by an audible alarm device. The electric circuit 35 iscompleted through the body 2 when the second limb 16 engages with thecontact 33 in the slot 32. While the fastener is unloaded the secondlimb 16 is spaced from the contact 33, at the opposite end of the slot.When load is applied on the fastener the second limb 16 moves along theslot toward the contact 33. When it engages with the contact 33, asindicated in phantom, the electric circuit 35 is completed and the bulb36 is illuminated to give warning that the loading on the fastener hasreached the proof lead.

The element, in either form described, the plug and scale, and disc, canbe made quite cheaply and can be assembled as necessary and fitted tothe body of the fastener without undue difficulty, so that the fastenercan be produced at a cost which is not excessively expensive comparedwith a conventional fastener.

We claim:
 1. A load indicating fastener comprising a body (2) having anabutment (9) which is moved by the changing length of the body (2) underan applied load, and load sensing means (10) comprising an element (11)disposed in the direction in which a load is applied to the body (2)when the fastener is in use characterised in that the element (11) is ofelongated U-shape, a first limb (13) of the U providing an anchoragepart (14) which is rigidly anchored to the body (2), the closed end ofthe U providing a bearing part (15) which bears on the abutment (9) soas to move with the abutment relative to the anchorage part (14) inresponse to a load applied to the body, and a second limb (16) of the Uhaving an indicator (17) at its outer end which responds to movement ofthe bearing part (15) and registers the applied load related to thatmovement on a scale (19).
 2. A load indicating fastener according toclaim 1 characterised in that the element (11) is made of a materialhaving the same coefficient of thermal expansion as the material of thebody (2).
 3. A load indicating fastener according to claim 2characterised in that the element (11) is made of wire.
 4. A loadindicating fastener according to claim 2 characterised in that the limbs(13,16) of the element (11) are resiliently movable relative to oneanother about the bearing part (15) in response to movement of thebearing part with the abutment (9).
 5. A load indicating fasteneraccording to claim 2 characterised in that at least in part the loadsensing means (10) is located in a blind bore or passage (7) in the body(2).
 6. A load indicating fastener according to claim 5 characterised inthat the abutment (9) is formed by the closed end of the bore or passage(7).
 7. A load indicating fastener according to claim 5 characterised inthat the abutment (9) is provided by an insert (27) in the closed endportion of the bore or passage (7).
 8. A load indicating fasteneraccording to claim 2 characterised in that the anchorage part (14) ofthe element is anchored to a support (12) fixed to the body (2) andclosing the bore or passage (7).
 9. A load indicating fastener accordingto claim 8 characterised in that the scale (19) is on the support (12).10. A load indicating fastener according to claim 8 or claim 9characterised in that the second limb (16) of the element (11) passesthrough an opening (18) in the support (12) to the scale (19).
 11. Aload indicating fastener according to claim 5 characterised in that theanchorage part (28) of the element (11) is anchored directly to the body(2).
 12. A load indicating fastener according to claim 11 characterisedin that the anchorage part (29) is in the form of a coil which isanchored in a mouth (8) of the bore or passage (7).
 13. A loadindicating fastener according to claim 2 characterised in that the loadsensing means (10) is adapted to give a signal that a given load orloads has or have been applied to the body (2) when the fastener is inuse.
 14. A load indicating fastener according to claim 13 characterisedin that the load sensing means (10) is contained in an electricalcircuit designed to produce an audible and/or visual signal when a givenload or loads is or are registered on the scale (19).
 15. A loadindicating fastener according to claim 13 characterised in that the loadsensing means (10) is contained in an electrical circuit adapted tooperate means for applying load on the body (2) in use of the fastener.16. A load indicating fastener according to claim 1 characterised inthat the element (11) is made of wire.
 17. A load indicating fasteneraccording to claim 1 characterised in that the limbs (13, 16) of theelement (11) are resiliently movable relative to one another about thebearing part (15) in response to movement of the bearing part with theabutment (9).
 18. A load indicating fastener according to claim 1characterised in that at least in part the load sensing means (10) islocated in a blind bore or passage (7) in the body (2).
 19. A loadindicating fastener according to claim 1 characterised in that theanchorage part (14) of the element is anchored to a support (12) fixedto the body (2) and closing the bore or passage (7).
 20. A loadindicating fastener according to claim 1 characterised in that the loadsensing means (10) is adapted to give a signal that a given load orloads has or have been applied to the body (2) when the fastener is inuse.